1. Field of the Invention
This invention relates generally to a system and method for operating a fuel cell system in a stand-by mode and, more particularly, to a system and method for operating a fuel cell system in a stand-by mode that includes determining when to enter the stand-by mode based on an optimization between fuel cell stack power and battery power and then providing a dynamic stand-by mode operation where the fuel cell stack is shut off and the cathode compressor operates at an idle speed until a calculated time has elapsed and then providing a static stand-by mode where the compressor is not operating.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. A typical cathode compressor will include air bearings. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between the two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
Most fuel cell vehicles are hybrid vehicles that employ a supplemental power source in addition to the fuel cell stack, such as a high voltage DC battery or an ultracapacitor. A bi-directional DC/DC converter is sometimes employed to match the battery voltage to the voltage of the fuel cell stack. The power source provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the fuel cell stack is unable to provide the desired power. The fuel cell stack provides power to an electrical traction motor through a DC high voltage electrical bus for vehicle operation. The battery provides supplemental power to the electrical bus during those times when additional power is needed beyond what the stack can provide, such as during heavy acceleration. For example, the fuel cell stack may provide 70 kW of power, however, vehicle acceleration may require 100 kW of power. The fuel cell stack is used to recharge the battery or ultracapacitor at those times when the fuel cell stack is able to provide the system power demand. The generator power available from the traction motor during regenerative braking is also used to recharge the battery or ultracapacitor.
It is necessary to provide control algorithms on a fuel cell hybrid vehicle to determine how much power will be provided by the fuel cell stack and how much power will be provided by the battery in response to a driver power request and under all vehicle operating conditions. It is desirable to optimize the power distribution provided by the fuel cell stack and the battery so that the amount of hydrogen used to operate the vehicle is minimized. In other words, it is desirable to operate the fuel cell system in the most efficient manner that allows the vehicle to travel the farthest distance using the least amount of hydrogen. The battery must be operated within a defined state-of-charge (SOC) range, where the control algorithms typically provide a SOC set-point to which the battery charge and discharge is controlled based on that set-point.
When a fuel cell system on a vehicle is in an idle mode, such as when the vehicle is stopped at a stop light, where the fuel cell stack is not generating power to operate system devices, air and hydrogen are generally still being provided to the fuel cell stack, and the stack is generating output power. This power is typically used to recharge the battery until an upper SOC limit of the battery is reached, where if the battery is charged beyond this upper limit, the battery may be damaged. When this SOC limit is reached, the battery load on the stack is removed, which increases the stack voltage, but causes certain phenomenon that decrease the life of the stack. If the fuel cell system is turned off during the idle condition, then the problem of providing a load on the stack when the battery has reached its maximum SOC does not need to be addressed. Also, providing hydrogen to the fuel cell stack when it is in the idle mode is generally wasteful because operating the stack under this condition is not producing very much useful work, if any.
For these and other fuel cell system operating conditions, it may be desirable to put the system in a stand-by mode where the system is consuming little or no power, the quantity of hydrogen fuel being used is minimal and the system can quickly recover from the stand-by mode so as to increase system efficiency and reduce system degradation. U.S. patent application Ser. No. 12/723,261, titled, Standby Mode for Optimization of Efficiency and Durability of a Fuel Cell Vehicle Application, filed Mar. 12, 2010, assigned to the assignee of this application and herein incorporated by reference, discloses one process for putting a fuel cell system on a vehicle in a stand-by mode to conserve fuel.